About this tool
The Mixing Ratio & Specific Humidity Calculator reports two closely related moisture measures used in meteorology and thermodynamics. The mixing ratio (w) is the mass of water vapor per kilogram of dry air, while specific humidity (q) is the mass of water vapor per kilogram of total (moist) air. Both are conserved as air rises or sinks without condensation, which is why forecasters and atmospheric scientists prefer them over relative humidity.
The tool first computes the actual vapor pressure e = 6.1094·exp(17.625·T/(T+243.04))·RH/100 in hPa, then applies w = 621.97 · e/(P − e) for the mixing ratio and q = 621.97 · e/(P − 0.378·e) for specific humidity, both in g/kg, where P is the station (barometric) pressure in hPa. The constant 621.97 = 1000 · Rd/Rv = 1000 · 0.62197 is the ratio of the gas constants for dry air and water vapor.
All math runs locally in your browser. Enter the temperature (°C or °F), relative humidity, and station pressure — which defaults to the standard sea-level value of 1013.25 hPa if left blank. Relative humidity outside 0–100%, a non-positive pressure, or a vapor pressure exceeding the station pressure are all rejected as invalid.